where foo is whatever library you need to link against. The include_dirs specifies where to find the ppport.h file. Also, the build-requires must be like:

build_requires => {
'ExtUtils::CBuilder' => 0,
},

Finally, unlike the old MakeMaker, XS modules should go in the right place in the lib/ tree, e.g. Foo::Bar should be lib/Foo/Bar.xs, there will probably also be a lib/Foo/Bar.pm already in that directory.